I am desigining a 6ft span R/C airplane required to carry a 2 kg payload with 25 mins loiter. Now i want to estimate its lift through CFD, since I can't any good maths for predicting Low Reynold Number lift.

can any any body tell me that how will i simulate the propeller effects on my R/c plane. The data i have is the prop diameter, RPM adn Power. Which B.C should i use and how to model that condition in my geometery!

wouldn't you want to make sure that at the specified RPM of the motor your prop blades are generating enough lift to pull the plane forward at the required speed necessary for the plane's wings to develop enough lift to make the airplane fly with the required load?

If that is the case, and I had to do it in Fluent, I would do a study of the lift generated by flow over the prop blade and find the necessary velocity to get the lift I needed. Unless I missed something, that is what I would do.

Actually i want to see that if my design is flown at a specific speed then how much lift will it generate ... and then if the propeller effects are added then how much will be the difference (I am assuming that my prop and rpm are generating enough thrust to fly at that specific speed)

do you have a cross-section of your wing? That would be the easiest way to do it. If it is a standard wing you can probably find a lot of data already dealing with that specific wing. You can also check your pitch etc.